In this study, a novel bi-order model combined with zero- and first-order kinetics was proposed,for the decomposition of PMMA (MW = 120,000 g/mol) in concentrated HNO3 by microwave irradiation. To develop and validate this model, Fourier Transform Infrared spectroscopy, scanning electron microscopy, fractional-life method, the gravimetric analysis and Newton's method were utilized. Rate constants, activation energies, the pre-exponential factors and the weight fractions (Phi) via main-chain scission for the decomposition at 423-453 K were derived from this model. The zero-order reaction was observed dominant at 423-443 K, while the first-order reaction dominated at 453 K and 473 K. The digestion efficiency increased as HNO3 was increased to >3 mL at 423 K-443 K. At 473 K. the digestion was almost 100% when HNO3 volume was >3 mL. The estimated Phi values increased with HNO3 volume at 423 and 443 K, but varied insignificantly at 453 and 473 K. (C) 2009 American Institute of Chemical Engineers AIChE J, 55: 2150-2158. 2009